Developmental changes in plasma membrane fluidity in chick embryo heart.

1. Decreases in the rate of transport of sugars (facilitated transport), amino acids (active transport), and urea (simple diffusion) occur in chick embryo heart during development. This work considers the possibility that changes in the plasma membrane fluidity during development contribute to the observed changes in transport activities. 2. Technics were developed for subcellar fractionation of chick embryos and adult chickens. 3. The depolarization of the fluorescence of 1,6-diphenylhexatriene was used to estimate the fluidity of the lipid portion of plasma membrane enriched fractions of hearts from chick embryos at various stages of development and from adult hearts. 4. There is a pattern of decreasing membrane viscosity as development proceeds. Between 5-6 days and 10 days of embryonic life a 20% decrease in viscosity of the plasma membrane-enriched fraction occurs. Between 10 and 20 days of embryonic life there is no significant change in viscosity. Between 20 days of development (1 day before hatching) and adulthood there is a further 55% decrease in plasma membrane viscosity. 5. It is proposed that the changes in membrane fluidity observed may contribute to developmental changes in membrane transport activities, but other factors must also be involved.